Since ectopic thyroid glands are usually located in the midline, cases of ectopic thyroid cancer occurring in the lateral neck are extremely rare. Ectopic thyroid carcinoma is a small carcinoma of the thyroid gland often diagnosed after the development of lymph node metastasis or distant metastasis, which is also very rare. In this article, we report a case of ectopic thyroid carcinoma arising from the lateral neck or occult thyroid carcinoma that was initially thought to be a lymph node metastasis, but proved difficult to diagnose. Preoperative contrast-enhanced computed tomography (CT) revealed partial thickening of the cyst wall, which led us to exclude not only ordinary lateral cervical cysts, but also lymph node metastasis from a malignant tumor and lymph node cyst. Therefore, we decided to submit the thickened part of the cyst wall for intraoperative rapid histopathology, and confirm the presence of malignant findings intraoperatively. Intraoperative rapid histopathology failed to reveal any evidence of malignancy, but the final histopathological diagnosis of the resected specimen was papillary thyroid carcinoma. The final pathological diagnosis of the excised specimen was papillary carcinoma of the thyroid gland. Even in consultation with the pathologist, it was difficult to diagnose thyroid cancer based on frozen-section histopathology alone, without prior information that the cancer originated from the thyroid gland. The results of the examination were explained to the patient and the usefulness of additional treatment, such as total thyroidectomy or right lobectomy, was explained to him; however, he refused to undergo such treatment and is being followed up as an outpatient. Although rare, the possibility of ectopic thyroid carcinoma or lymph node metastasis from an occult thyroid carcinoma should be borne in mind in the differential diagnosis of masses resembling lateral cervical cysts, and careful attention should be paid during the preoperative examination.
